Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Examples of wireless communication systems are Long Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS) and Global System for Mobile communications (GSM).
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, machine to machine devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. Data transmission in LTE is controlled by the radio base station.
One function of 3GPP cellular radio technologies is the control of user mobility by using the Radio Resource Control (RRC) and X2AP protocols. The network controls the handover of UEs in RRC Connected mode from one cell to another, whereas a UE in Idle mode performs cell selection and reselection itself. Embodiments herein are illustrated with examples from the Evolved Universal Terrestrial Radio Access (EUTRA) also known as LTE system.
When the UE is handed over from a source cell to a target cell, handover preparation is needed. Upon handover preparation the target radio base station or eNB is provided the current UE RRC configuration comprised in the handover preparation information message (the HandoverPreperationInformation message, for example as described in technical Specification TS36.331, version 12.1.0). This message is used to transfer the E-UTRA RRC information used by the target eNB during handover preparation, including UE capability information. The target eNB decides the RRC configuration after the handover and therefore a RRC configuration message is transparently sent to the UE via the source eNB as an octet string comprised in the handover command. Normally full configuration is required if the target eNB has a different Access Stratum (AS) version than the source eNB.
If the RRC configuration in the handover preparation message is incomplete or the source cell has configured the UE with a RRC protocol version that is not comprehended by the target eNB, the target eNB typically performs a full configuration (if possible), i.e. performs a reconfiguration from scratch. Otherwise the target eNB may modify or maintain the current RRC configuration.
3GPP has recently agreed within the scope of low-cost Machine Type Communication (MTC) work item to introduce a new UE category termed as a “low complexity category” (which may also be referred to as category 0, or category 11). A draft Change Request (CR) for the introduction of the low complexity category is available in R2-140964 as presented during the RAN2#85 meeting from 10.02.2014 to 14.02.2014, in Prague, Czech Republic. A problem arises, since this new category is less capable than legacy UE categories, for example, legacy category 1. Furthermore, the legacy categories (including categories 1, 2, 3, 4 and 5 that were first introduced with Release 8 of the RRC protocol), such as category 1, are mandatory to signal even though the low complexity category UE does not have any such category, and thus full configuration will always be required from the legacy eNB because it has a different AS version than the source eNB. In other words, since a legacy UE-Category field is mandatory required to be present in the UE-EUTRA-Capability container, omitting such a mandatory field results in a decoding error, and therefore to avoid this all UEs shall indicate one category in the mandatory field even if they do not support any such category.
Accordingly, upon handover preparation, a legacy target eNB that does not comprehend the new low complexity category may erroneously assume that the UE may be successfully configured with the indicated legacy category, for example a legacy category such as category 1. Consequently, the handover fails and the UE experiences a radio link failure. Subsequently, the UE tries to trigger RRC Connection Re-establishment which also fails. Finally the UE transitions to Idle mode where it searches for suitable cells.
This has the disadvantage of causing unnecessary signaling due to the failed handovers and the unsuccessful RRC connection re-establishment requests, which results in the inefficient use of bandwidth.